The current studies deal mainly with the synthesis and regulation of the components of cytoplasmic and mitochondrial ribosomes in hybrid and other mammalian cells. Hamster-mouse hybrids synthesizing mostly mitochondrial RNA (mt-rRNA) of one species, contained mostly mtDNA of the same species. Hybrid 10A has a predominantly mouse nuclear genome, synthesizing mainly mouse cytoplasmic rRNA (cyt-rRNA) and a predominantly hamster mitochondrial genome, synthesizing mainly hamster mt-rRNA. After infection with adenovirus type 2, the synthesis of cyt- rRNA was inhibited in hamster cells but not in mouse cells; the total cyt-rRNA synthesis of hybrid 13B was inhibited but its hamster/mouse ratio did not change. When hybrid B13 was incubated for a short or long time with (methyl-H3) methionine or for a long time with (H3)uridine, about 80% of the labeled cyt-rRNA was of hamster type. We postulate that a temporary difference in the specific activity of (H3)UTP in possibly segregated mouse and hamster types of nucleoli might account for these results. The kinetics of isotope accumulation into individual cytoplasmic ribosomal proteins (r-proteins) is being studied in HeLa by pulse-chase experiments and two-dimensional gel electrophoresis (2DGE). As qualitative differences between hamster and mouse r-proteins are found by 2DGE, we are now studying the radioactive b-proteins of hybrid B13. We recently found in mammalian cells, two methylated, cytoplasmic RNAs, of low molecular weight, with half lives of about 10 minutes, apparently coded by the nuclear genome, with a very brief nuclear processing step, if any, and which are not part of the S45 precursor rRNA transcript.